An accelerometer is one of the primary sensors used in on-board automotive safety control systems and navigational systems, particularly crash sensing systems. Examples of such automotive applications include anti-lock braking systems, active suspension systems, supplemental inflatable restraint systems such as air bags, and seat belt lock-up systems. An accelerometer is a device which measures acceleration, or more accurately, accelerometers measure the force that is exerted by a body as the result of a change in the velocity of the body. A moving body possesses inertia which tends to resist the change in velocity. It is this resistance to any change in velocity that is the source of the force which is exerted by the moving body. This force is directly proportional to the acceleration component in the direction of movement when the moving body is accelerated.
In one conceptual form of a conventional accelerometer, a mass is suspended between two spring members which are coaxially attached on opposite sides of the mass. The mass is maintained in a neutral position so long as the system is at rest or is in motion at a constant velocity. When the mass-spring support system undergoes a change in velocity in the direction of the springs' axis, i.e. an acceleration or deceleration parallel the spring axis, the spring mounted mass will resist the movement because of its inertia. This resistance to the change in velocity will force one of the springs to be in tension while the other spring is compressed. Accordingly, the force acting on each spring is equal but opposite in magnitude to the force acting upon the other.
In a micromachine accelerometer employing iezoresistive microbridges, acceleration in the plane perpendicular to a plane through the proof mass and microbridges can be detected. It causes a compressive or tensile load on portions of the oppositely disposed piezoresistive microbridges supporting the proof mass, depending on which direction it comes from in that plane. It is the accelerating force on the support system for the proof mass and the proof mass inertia which generates compressive or tensile loads on the piezoresistive microbridges. In turn, the resulting compressive or tensile loads change electrical resistance of piezoresistors in the microbridges. This change in electrical resistance can be sensed to determine the magnitude of the acceleration component perpendicular to the plane of the common axis shared by the pair of piezoresistive microbridges. This type of piezoresistive microbridge accelerometer is attractive for precision measurements.
Such precision products could be quite useful in automotive applications. However, they must be adequately packaged to protect the micromachined accelerometer from an automobile's harsh environment. The accelerometer not only should be isolated from the mechanical stresses associated with mounting the package, but also must be protected from the extraneous road and vehicle vibrations during use. Moreover, the package must isolate the accelerometer from the harsh automotive environment, such as salt, grease, dust and moisture. In addition, it should be easy to assemble, to improve quality and durability and to reduce cost. Still further, the proof mass, microbridges and their supporting system have to be packaged, and/or the package itself supported, in a manner wherein the microaccelerometer will only be sensitive to those particular external forces which it is intended to detect. For example, if it is to detect acceleration or deceleration of the vehicle itself, one would not ordinarily want it to also detect jarring of the vehicle due to a bump or chuck hole in a road surface. In addition, it would be desirable if the packaging were small and compact while also providing an inert protective atmosphere for the micromachined accelerometer. Lastly, it would also be advantageous if such a packaging means facilitated early testing of the device while also being amenable to high volume, low cost automotive production techniques.
Therefore, it would be advantageous to provide microaccelerometer packaging which is constructed so as to isolate the micromachined accelerometer from mechanical stresses associated with mounting the device and extraneous vibrations, while also providing a testable assembly which is small, compact and amenable to automotive production techniques.